1. Field of the Invention
The present invention relates generally to melanin compositions suitable for use as ultraviolet radiation absorbents. More particularly, the present invention relates to such compositions, methods for their preparation, and methods for their use, where the melanin is color modified to enhance its suitability for incorporation into cosmetic and other topically-applied formulations.
With the growing concern over the deleterious effects of sunlight on human skin, a number of ultraviolet radiation absorbent substances have been identified and developed for use in topical sunscreen compositions. In general, the substances function by absorbing radiation in the ultraviolet radiation region and re-emitting the absorbed energy in other, less harmful radiation spectra.
Although generally effective, most of the sunscreen substances which are presently in widespread use suffer from a number of drawbacks. In particular, most of the sunscreen substances are very oily and are difficult to formulate into suitable products. Such sunscreen substances may also be incompatible with other ingredients in a cosmetic preparation where it would be desirable to incorporate sunscreen protection. Additionally, many sunscreen compositions are characterized by strong odor, usually requiring the addition of a fragrance in order to formulate a commercially-acceptable product. Furthermore, sunscreens are easily lost from the skin either by rubbing or by intradermal penetration. Finally, many of the widely used sunscreen substances can be toxic at high concentrations when absorbed through the skin.
For these reasons, it has been proposed to use melanins as the active ingredient in a wide variety of sunscreen and cosmetic formulations. Melanins are a major class of broad-spectrum ultraviolet-absorbing organic polymers found naturally in the human epidermis and elsewhere. The use of melanins as the ultraviolet-absorbent substance in sunscreen and cosmetic formulations would have a number of advantages over the prior art compositions discussed above. They are natural substances and substantially non-toxic, even when applied at very high concentrations to the skin. They are free from objectionable odor and appear to be compatible with a wide variety of formulations, including both primary sunscreen formulations and cosmetic formulations. Finally, melanins are not volatile and will not be lost from the skin through either evaporation or penetration.
In particular, it has been proposed to incorporate melanin pigments within polymeric particles in order to form melanin aggregates which may then be combined in cosmetic preparations and other formulations suitable for topical application. The incorporation of melanin pigment within the polymeric particles appears to enhance the ability of the melanins to absorb ultraviolet radiation so that such compositions afford improved protection from the sun when applied topically to the skin.
Unfortunately, most melanins are darkly colored, typically being black, dark brown, reddish brown or the like. The incorporation of such colored melanins into lightly colored cosmetic formulations can thus have an undesirable aesthetic impact. In particular, many cosmetic formulations are desired to have a pure white appearance, where the incorporation of a substantial amount of colored melanin can have a substantial deleterious effect. The incorporation of the darkly colored melanins in polymeric particles does little to improve the situation. The melanins can impart a dark hue to the polymeric particles, which in turn have an adverse effect on the color of the cosmetic formulation.
For these reasons, it would be desirable to provide color-modified melanins having a much lighter color which is compatible with the coloring of a wide variety of cosmetic formulations, particularly being compatible with white or other light colored formulations. The color modification of the melanins, however, should have little or no adverse impact on the ability of the melanins to absorb ultraviolet radiation when incorporated into the cosmetic formulations.
2. Description of the Background Art
The mechanism underlying the bleaching of melanin in hair is described in Wolfram et al. (1970) J. Soc. Cosmet. Chem. 21:875-900. The treatment of melanin pigment with oxidizing and reducing agents was generally found to induce no apparent physical change in the melanin, although treatment with hydrogen peroxide was found to cause disintegration of melanin granules. The use of solubilized melanin in cosmetic formulations is described in U.S. Pat. No. 4,806,344. The incorporation of melanins into polymeric substrates, such as plastic lenses, is described in U.S. Pat. No. 4,698,374. The incorporation of melanins into polymeric particles which in turn may be incorporated into cosmetic formulations is disclosed in U.S. Patent No. 4,806,360.